Sources of nitrogen and sulfur in wet deposition at Guiyang, southwest China

Whilst limited information on nitrogen and sulfur isotopic composition in wet deposition in cities is available, very few data on the sources of sulfur and nitrogen in both light rainfalls (< 5 mm in precipitation and < 4 h in duration) and heavy rainfalls ( > 100 turn in precipitation and < 24 h in duration, caused by typhoon from the Pacific Ocean) have been accumulated. For both species a significant difference in mean sulfur and nitrogen isotopic composition was reported in this study between samples collected from light rainfalls (-4.9 +/- 2.8parts per thousand for sulfate and + 2.0 +/- 4.4parts per thousand. for nitrate) and heavy rainfalls ( + 4.6 +/- 5.0parts per thousand for sulfate and + 4.1 +/- 4.0parts per thousand for nitrate), supporting that sulfur and nitrogen in the two types of rainfalls are of different sources. For sulfate, the negative delta(34)S values in light rainfalls are associated with local sources (coal combustion and biogenic sulfur) while sulfur in heavy rainfalls, with positive delta(34)S values, is of maritime origin. For nitrate, there is no definite information on the sources of nitrogen in light rainfalls because of a wide range of delta(15)N values (- 3.8parts per thousand to + 10.7parts per thousand), but the higher delta(15)N values ( > + 6.0parts per thousand) are probably ascribed to dry deposition or coal-fired power plants; the mean delta(15)N value of + 4.1parts per thousand in heavy rainfalls may also be a reflection of maritime origin. Measurements of delta(15)N values for ammonium in light rainfalls show that delta(15)N values are closely related with ammonium concentrations (R-2 = 0.92). The lower values in the range of delta(15)N values (-1.7parts per thousand to -22.0parts per thousand) for ammonium are due to less incorporation of N-15 by cloud water, which probably has a delta(15)N value of about -22.0parts per thousand. But the higher concentrations (averaging 1.25mgl(-1)) and lower delta(15)N values (averaging -12.2+/-6.7parts per thousand) indicated that ammonium steps from excretory wastes are extensively applied in agriculture and NH3 released from soils. (C) 2002 Elsevier Science Ltd. All rights reserved.